Engines operating on gaseous fuels, such as natural gas, are commonly supplied with a lean fuel mixture, which is a mixture of air and fuel containing a relatively high ratio of air to fuel. The lean fuel mixture often results in misfires, detonation, incomplete combustion and poor fuel economy. One factor that can lead to such events is the poor ability of conventional spark plugs to effectively ignite a lean fuel mixture in the cylinder of the operating engine. More effective combustion of lean fuel mixtures can be achieved using a pre-combustion chamber.
Pre-chamber (i.e., pre-combustion chamber) spark plugs are used in the pre-combustion chamber and are typically used to enhance the lean flammability limits in lean burn engines such as natural gas lean burn engines. In known pre-chamber spark plugs such as the pre-chamber spark plug disclosed in U.S. Pat. No. 5,554,908, the spark gap is confined in a cavity having a volume that is typically less than three percent of the engine cylinder displacement. The top portion of the cavity is shaped as a dome and has various tangential induction/ejection holes. During operation, as the engine piston moves upward during the compression cycle, air/fuel is forced through the induction holes in the pre-chamber. The orientation of the holes creates a swirling motion inside the pre-chamber cavity.
The difference in density between the air and the fuel in conjunction with the swirl motion causes fuel stratification within the pre-chamber cavity. With proper location of the spark gap, effective ignition can be achieved in a fuel rich area. The fast burning of fuel in the pre-chamber cavity can result in highly penetrating jets of flames into the engine combustion chamber. These jets of flames provide the ability to achieve a more rapid and repeatable flame propagation in the engine combustion chamber at leaner air/fuel mixtures.
One problem that the prior art does not address is spark plug operation with ultra-lean air/fuel mixtures (lambda>1.75) and high BMEP (Brake Mean Effective Pressure) (>18 bars). At such operating conditions, the spark plug life tends to be very short. As a result, commercialization of high efficiency and high power density gas engines is not practical.
What is not described in the prior art are the attributes and configurations required for the pre-chamber cavity, the induction/ejection holes, the shape and location of electrodes that minimize electrode erosion and maximize spark plug life, especially with ultra-lean air/fuel mixtures and high BMEP. The prior art also does not address the issue of water condensation inside the spark plug pre-chamber and in between the electrodes causing short circuit and plug misfire. Additionally, the prior art does not address the issue of plug surfaces overheating and causing preignition.
The invention provides such attributes and configurations for engines operating with ultra-lean air/fuel mixtures and high BMEP. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.